The keystone is the last stone that they place into an arch, it makes the structure self supporting. It bears the less pressure compared to the other wedge shaped stones therefore the "key" word is very appropiate. Due to it's importance it is often carved and highlighted, bigger than the other stones or slightly dropped. In the medieval times masons mixed blood into the cement when they placed the keystone into the arch. It was a sacrifice to insure the stability of the building.
The black ring is carved from ardesia, a common stone around here. The white ones are Carrara marble, while the red one is Hungarian red marble.
The keystone's colour referes to the traditional blood offering.
The diameter of the circle (with the silver rim) is 32 mm.
The length of the sterling silver chain is 43 cm (16.9 inches)
Carrara marble, Hungarian red marble, cement, sterling silver